1. Field of the Invention
The present invention relates to an image forming apparatus to be used as a color copying machine and a color printer, and to an image forming method using the image forming apparatus.
2. Description of the Related Art
According to an earlier development, an image forming apparatus of so-called the tandem system, that forms color toner images for each colors separately on different photosensitive members (hereinafter also referred to simply as “the photosensitive member”) and then interposes these color toner images onto an intermediate transfer member or a recording material to form images, has been known as one for forming color images. (See, for example, Japanese Patent Laid-open No. 2001-222129).
However, this tandem system is a system to form color images by forming electrostatic latent images on separate photosensitive bodies in accordance with color-separated image information, that is, image information corresponding to each colors of yellow, magenta, cyan and black, respectively, then forming color toner images corresponding to each colors of yellow, magenta, cyan and black, and subsequently interposing these toner images onto an intermediate transfer member or a recording material. This color image forming apparatus of the tandem system is adopted to form color images by interposing toner images with different color hues formed by a plurality of image forming units onto an intermediate transfer member or a recording material. Therefore, it is feasible to develop a high-speed image forming apparatus of the electrophotography system capable of forming color images at a high speed based on the image forming apparatus of the tandem system.
However, since photosensitive bodies are separately used for color toner images for each colors in the tandem system, there is a tendency that uneven development of colors and deviation in colors are easily caused unless the performance of the respective photosensitive bodies is stabilized.
On the other hand, a technology to form fine latent images on a photosensitive member by using a light source for exposure having a small spot diameter to thereby form images with fine dots has been developed for aiming at improving color images so that they are furnished with high image qualities. As an example, a method to use a light source of which spot diameter is 4,000 μm2 or less to form highly-fined latent images on a photosensitive member is known (See, e.g., Japanese Patent Laid-open No. H8-272197). When such an exposure system with a small spot diameter is used for the image forming by means of the tandem system, it is required that the performance of the photosensitive bodies respectively corresponding to the above-mentioned four colors must have been stabilized.
Besides, when an organic photosensitive member is used in the image forming apparatus of the tandem system, the attrition of a photosensitive member for black, that forms images of characters too, is greater than the photosensitive bodies for yellow, magenta and cyan. In case of an image forming unit in which the respective photosensitive bodies for Y (yellow), M (magenta), C (cyan) and Bk (black) integrally included, the life of the image forming unit is determined in accordance with the life of the photosensitive member for black. This leads to a problem that the photosensitive bodies for the other colors are obliged to be replaced before they reach to their lives for use. In order to solve such a problem, an image forming apparatus of the tandem system, in which only the photosensitive member for black is formed of an amorphous silicon-based photosensitive member and the photosensitive bodies for the other colors are formed of organic photosensitive bodies, has been proposed (Seem, e.g., Japanese Patent Laid-open No. 2002-372820). When an amorphous silicon-based photosensitive member is used only for black, the difference in the attrition of the film thicknesses of the photosensitive bodies between the amorphous silicon-based and the organic photosensitive bodies will be reduced. However, there is a remarkable difference in the adherence of a toner to a photosensitive member depending upon if it is an amorphous silicon-based photosensitive member or an organic photosensitive member. Therefore, when the both types of photosensitive bodies are used at the same time, great differences may be caused in the development performance, transfer performance and other performances of between the toners, which lead to a problem that it will be difficult to obtain a stable image quality for both of images of characters and color images.
For example, due to the dispersion in the qualities of the manufactured toners, the variation in the sizes particularly of black dot images becomes greater than color dot images. Accordingly, there is a problem of causing insufficient definition in color images.
Furthermore, since there is a difference in the toner filming performance onto the surface of a photosensitive member between an amorphous silicon-based photosensitive member and an organic photosensitive member, there is another problem that both qualities of images of characters and color images deteriorate during the use of toners of the similar properties for a long time.
Although an example of the tandem system that is furnished with different photosensitive bodies dedicated for each of the four colors was given above, the relevant problems may be caused even the case of using a single amorphous silicon-based photosensitive member common to the four colors. Specifically, in case of using a single amorphous silicon-based photosensitive member, the characteristics of each toner for a different color varies from one another on the same amorphous silicon-based photosensitive member, whereby a problem of causing dispersion of colors and so on has come up. Still further, in case of using a single amorphous silicon-based photosensitive member, the deterioration of the photosensitive member becomes greater because the photosensitive member is commonly used for all colors, and there is a tendency that the dispersion of colors becomes further greater.
The problems as described above are raised when using the amorphous silicon-based photosensitive member, or jointly using the amorphous silicon-based photosensitive member and the organic photosensitive member in an image forming apparatus of the tandem system, or solely using a single amorphous silicon-based photosensitive member. However, in actuality, countermeasures for solving the above-described problems have not been well-studied yet.